Preheat oven to 400°. Unfold puff pastry. Using a 4-in. round cookie cutter, cut out 4 circles. Place 2 in. apart on a greased baking sheet. Bake until golden brown, 14-16 minutes., Meanwhile, shred beef with 2 forks; transfer to a large saucepan. Add tomatoes, vegetables and pepper; bring to a boil. In a small bowl, mix cornstarch and water until smooth; stir into beef mixture. Return to a boil, stirring constantly. Cook and stir until thickened, 1-2 minutes., Ladle stew into 4 bowls; top each serving with a pastry round.